Global and domestic demand for fossil fuels continues to rise despite price increases and other economic and geopolitical concerns. As such demand continues to rise, research and investigation into finding additional economically viable sources of fossil fuels correspondingly increases. Historically, many have recognized the vast quantities of energy stored in oil shale, coal and tar sand deposits, for example. However, these sources remain a difficult challenge in terms of economically competitive recovery. Canadian tar sands have shown that such efforts can be fruitful, although many challenges still remain, including environmental impact, product quality, production costs and process time, among others.
Estimates of world-wide oil shale reserves range from two to almost seven trillion barrels of oil, depending on the estimating source. Regardless, these reserves represent a tremendous volume and remain a substantially untapped resource. A large number of companies and investigators continue to study and test methods of recovering oil from such reserves. In the oil shale industry, methods of extraction have included underground rubble chimneys created by explosions, in-situ methods such as In-Situ Conversion Process (ICP) method (Shell Oil), and heating within steel fabricated retorts. Other methods have included in-situ radio frequency heating (microwaves), and “modified” in-situ processes wherein underground mining, blasting and retorting have been combined to make rubble out of a formation to allow for better heat transfer and product removal.
Among typical oil shale processes, all face tradeoffs in economics and environmental concerns. No current process alone satisfies economic, environmental and technical challenges. Moreover, global warming concerns give rise to additional measures to address carbon dioxide (CO2) emissions that are associated with such processes. Methods are needed that accomplish environmental stewardship, yet still provide high-volume cost-effective oil production.
Below ground in-situ concepts emerged based on their ability to produce high volumes while avoiding the cost of mining. While the cost savings resulting from avoiding mining can be achieved, the in-situ method requires heating a formation for a long period of time due to the extremely low thermal conductivity and high specific heat of solid oil shale. Perhaps the most significant challenge for any in-situ process is the uncertainty and long-term potential of water contamination that can occur with underground freshwater aquifers. In the case of Shell's ICP method, a “freeze wall” is used as a barrier to keep separation between aquifers and an underground treatment area. Long-term prevention of contamination has yet to be conclusively demonstrated and there are few remedies should a freeze wall fail, so other methods are desirable to address such environmental risks.
One method and system that addresses these problems is disclosed and claimed in U.S. application Ser. No. 12/028,569, filed Feb. 8, 2008 which is incorporated herein in its entirety by reference. In that application, a method of recovering hydrocarbons from hydrocarbonaceous materials is disclosed including forming a constructed permeability control infrastructure. This constructed infrastructure defines a substantially encapsulated volume. A mined hydrocarbonaceous material, such as oil shale, can be introduced into the control infrastructure to form a permeable body of hydrocarbonaceous material. The permeable body can be heated sufficient to reform and remove hydrocarbons therefrom leaving a lean shale or other earthen material. During heating the hydrocarbonaceous material can be substantially stationary. Removed hydrocarbons can be collected for further processing, use in the process as supplemental fuel or additives, and/or direct use without further treatment. The lean shale or other material may remain in the infrastructure. The control infrastructure can include fully lined impermeable walls or impermeable sidewalls with a substantially impermeable floor and cap.
It has been recognized by the present inventors that a potential drawback to this method and system lies in the subsidence of the hydrocarbon lean materials within the infrastructure over time causing the cap and any overburden to settle below the initial grade, potentially to the extent of creating a concave surface. Settling below grade of the infrastructure may be undesirable from an environmental or reclamation point of view. Further, materials surrounding the capsule often possess minimal or no tensile strength. These materials may be placed in tension parallel to the capsule crown surface if the capsule surface settles below a horizontal plane to produce an increasingly concave surface and may subsequently separate or rupture as the cap of the infrastructure settles causing exposure of the contents within the infrastructure to the outside environment. Exposure of lean shale or other earthen materials, which may contain minimal amounts of unremoved hydrocarbons, heavy metals and the like may be undesirable. Also, gases trapped within the infrastructure or which may later vaporize may also be released.
For these and other reasons, the need remains for methods and systems which can enable improved recovery of hydrocarbons from suitable hydrocarbon-containing materials while providing for encapsulation and containment of the hydrocarbon lean or and other earthen materials that are subject to subsidence while avoiding below grade settling of the cap or cover of the infrastructure and overburden.